For the Bachmann, I'd use NCE's BACHDSL decoder, since it's a straight substitute for the light board on most RTR Bachmann diesels. You simply remove the loco shell, unsolder the 6 wires from the light board, be sure to pay attention to where the wires went. The instructions that come with the BACHDSL decoder are very straight forward and simple to follow, they will tell you where exactly to solder the wires. Another good thing about this decoder is that it comes with LED's, so you can get rid of the bulbs that come with the stock light board.
For the Athearn, I'd go with the Digitrax DH123AT or DH163AT that was suggested above. If you don't mind doing some soldering, then your decoder selection greatly increases for the Athearn.
TONY
"If we never take the time, how can we ever have the time." - Merovingian (Matrix Reloaded)
Digitrax DH123AT or DH163AT for the Athearns. Easy install with no soldering, unless you want to replace the "light up the entire front end" bulb with something that lights up only the headlight.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Thanks for your help David